1. Field of the Invention
The present-invention relates to light transmitting fibers such as low melting temperature fibers fabricated from polymers (e.g., polymethyl methacrylate, PMMA, etc.) which have been widely used in light transmission and illumination for industrial and medical applications. More particularly, the present invention relates to the power handling capabilities of such light transmitting fibers.
2. Description of the Background Art
The following description of the background art represents the inventors' knowledge and not necessarily knowledge of those within the art.
Low melting temperature fibers fabricated from polymers have advantages of high numerical apertures, mechanical flexibility, and low cost. Light is normally coupled directly into these fibers from a focused light source or from another fiber or fiber bundle by proximity coupling ("butt coupling").
Due to the low melting temperature of such polymers and their light absorbing properties, the power handling capability of a polymer fiber is relatively low. A critical parameter that restricts the total amount of light capable of being transmitted is the increase in temperature resulting from absorption within the fiber which, though negligibly small, causes "thermal run-away" and melting of the polymer fiber. The causes for the endface of the fiber gradually melting include not only light absorption but also non-transmitted light which is converted into heat as the result of mismatches in size and numerical aperture between the light source and the polymer fiber.
Light absorption is local power intensity--dependent. As a result, the shape of the intensity profile, which is typically Gaussian, is a major determinating factor for the input power at which a polymer fiber melts. In addition, if appropriate light-filtering is not employed to remove UV and IR light, the fibers will discolor and degrade in their transmission abilities. The latter is accelerated by high light intensity and heat generated from non-transmitted light.
Improvements can be achieved by adding a spatial filter between the light source and the polymer fiber. The spatial filter can consist of a heatsink and an aperture placed at the input interface of the polymer fiber and light source, or of a short piece of glass or quartz fiber, having a numerical aperture equal to or smaller than that of the polymer fiber and a length sufficient to remove unguided modes of light. The latter improves the performance in power transmission of a polymer fiber from the 100 mW range to the range of 300 to 400 mW for a 1-mm diameter PMMA fiber, for example. Nevertheless, the low melting temperature of such polymer fibers severely restricts the maximum power able to be coupled into a single polymer fiber or fiber bundle--as compared with a similar glass or quartz core fiber. For larger diameter single fiber typically 3 mm or larger, it is known in the art to interpose a glass fiber bundle between the light source and the polymer fiber. U.S. Pat. No. 4,986,622 (Martinez) teaches the use of a glass fiber bundle to transmit light from a light source to a bundle of plastic polymer fibers. Although it is known that such spatial filters increase the light able to be transmitted through a plastic fiber without degradation, the degree of spatial filtering is limited to eliminating unguided modes without modifying the shape of the Gaussian profile.
Although spatial filtering eliminates unguided modes, the input light has a Gaussian-like intensity profile for which the peak power is highest in the center and lowest at the perimeter of the beam. As a result, the intensity at the center of the end face of the polymer fiber becomes the limiting factor in the power handling capability of the fiber since high peak power above the absorption threshold will cause degradation of the fiber.